Heat the oil in a small saucepan, add the onion and cook for at least 10 minutes or until onions are soft. Put into a large bowl and leave to cool.
Once the onions have cooled add mince, parsley, Worcestershire sauce, tomato sauce and a sprinkle of salt.
Squash the mince together to mix all the ingredients. Make into 4 large balls, then press into burger shapes, sprinkle a little salt on top of each burger, cover with cling film and chill for at least an hour.
Preheat the grill, then grill the burgers for 15 minutes on one side, then on the same tray add the bacon, turn over the burgers, and cook for another 10-15 minutes, turning bacon over once also. All of the bacon fat will add extra flavour to the burgers.
Place the burgers and bacon onto kitchen paper, to drain off excess fat.
Place a cheese slice onto each bun, then the burgers, a small squirt of tomato sauce, another cheese slice, then top with bacon.